We are continuing work on the interactions between the adenylate energy charge and other metabolic parameters in the regulation of metabolism. In Escherichia coli, growth occurs only when the energy charge is in the normal range (0.8 to 0.9), but the adenylate pool (and the concentration of ATP) may fall by a factor of 3 with little effect. The properties of adenylate deaminase seem to guard the energy charge against sharp fluctuations by removing AMP when the charge falls, but its action stops short of depletion of the pool because of a high-order dependence on the concentration of ATP. Transfer of phosphoryl groups from ATP to the other nucleotide pools, which is essential to growth, is sharply controlled by energy charge. Work on this enzyme is continuing, and study of regulation of enzymes that catalyze reactions utilizing UTP, GTP, and CTP is being initiated. UDPglucose synthase appears to respond primarily to the UDPG/UTP ratio. The relation between the adenylate energy charge and the triphosphate/diphosphate ratio in the other pools is being studied in rat liver in situ.